After rewatching ESB this weekend and the IMPS movie, I was wondering - How exactly are At-Ats deployed? How do they get from ship to planet side? Any theories?

Well, I know the AT-STs can be carried from an Imperial shuttle. In Rogue Squadron II for GameCube, you see one dropping some kind of box which the walker emerges from. Obviously AT-ATs are much bigger, so they would need a larger ship.

According to the Star Wars Technical Commentaries, they are deployed from landing barges, but if they are in controlled territory, they can be shipped in parts and assembled:

http://www.theforce.net/swtc/walkers.html#deployment

Thanks, this was what I was looking for.